1. Field of the Invention
The present invention relates generally to a golf ball having a cover material made from a polyurea or polyurea/urethane hybrid composition. More particularly, the polyurea or polyurea/urethane is the reaction product of a blend of isocyanates, polyamines, and amine or hydroxyl curing agent. The resulting cover material has many advantages including improved thermal stability, durability, toughness, and cut/tear-resistance.
2. Brief Review of the Related Art
Multi-piece solid golf balls having an inner core and outer cover with an intermediate layer disposed there between are popular today in the golf industry. The inner core is made commonly of a rubber material such as natural and synthetic rubbers, styrene butadiene, polybutadiene, poly(cis-isoprene), or poly(trans-isoprene). Often, the intermediate layer is made of an olefin-based ionomer resin that imparts hardness to the ball. These ionomer acid copolymers contain inter-chain ionic bonding, and are generally made of an α-olefin such as ethylene and a vinyl comonomer having an acid group such as methacrylic, acrylic acid, or maleic acid. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. Commercially available olefin-based ionomer resins are used in different industries and include numerous resins sold under the trademarks, Surlyn® (available from DuPont) and Escor® and Iotek® (available from ExxonMobil), Amplify IO® (available from Dow Chemical) and Clarix® (available from A. Schulman). Olefin-based ionomer resins are available in various grades and identified based on the type of base resin, molecular weight, and type of metal ion, amount of acid, degree of neutralization, additives, and other properties. The outer cover of conventional golf balls are made from a variety of materials including olefin-based ionomers, polyamides, polyesters, and thermoplastic and thermoset polyurethane and polyurea elastomers.
In recent years, there has been high interest in using thermoset, castable polyurethanes and polyureas to make core, intermediate, and/or cover layers for the golf balls. Basically, polyurethane compositions contain urethane linkages formed by reacting an isocyanate group (—N═C═O) with a hydroxyl group (OH). Polyurethanes are produced by the reaction of a multi-functional isocyanate with a polyol in the presence of a catalyst and other additives. The chain length of the polyurethane prepolymer is extended by reacting it with a hydroxyl-terminated curing agent. Polyurea compositions, which are distinct from the above-described polyurethanes, also can be formed. In general, polyurea compositions contain urea linkages formed by reacting an isocyanate group (—N═C═O) with an amine group (NH or NH2). The chain length of the polyurea prepolymer is extended by reacting the prepolymer with an amine curing agent. Hybrid compositions containing urethane and urea linkages also may be produced. For example, a polyurea/urethane hybrid composition may be produced when a polyurea prepolymer is reacted with a hydroxyl-terminated curing agent as discussed further below.
Golf ball covers made from polyurethane and polyurea compositions are generally known in the industry. In recent years, polyurethane and polyurea cover materials have become more popular, because they provide the golf ball covers with a desirable combination of “hard” and “soft” features. The relative hardness of the cover protects the ball from being cut, abraded, and otherwise damaged. In addition, such harder-covered golf balls generally reach a higher velocity when struck by a club. As a result, such golf balls tend to travel a greater distance, which is particularly important for driver shots off the tee. Meanwhile, the relative softness of the cover provides the player with a better “feel” when he/she strikes the ball with the club face. The player senses more control over the ball as the club face makes impact. Such softer-covered balls tend to have better playability. The softer cover allows players to place a spin on the ball and better control its flight pattern. This is particularly important for approach shots near the green. Polyurethane and polyurea covered golf balls are described in the patent literature, for example, U.S. Pat. Nos. 5,334,673; 5,484,870; 6,476,176; 6,506,851; 6,867,279; 6,958,379; 6,960,630; 6,964,621; 7,041,769; 7,105,623; 7,131,915; and 7,186,777.
As discussed above, isocyanates with two or more functional groups are essential components in producing polyurethane and polyurea polymers. These isocyanate materials can be referred to as multi-functional isocyanates. Such isocyanates can be referred to as monomers or monomeric units, because they can be polymerized to produce polymeric isocyanates containing two or more monomeric isocyanate repeat units.
Aromatic isocyanates are normally used for several reasons including their high reactivity and cost benefits. Examples of conventional aromatic isocyanates include, but are not limited to, toluene 2,4-diisocyanate (TDI), toluene 2,6-diisocyanate (TDI), 4,4′-methylene diphenyl diisocyanate (MDI), 2,4′-methylene diphenyl diisocyanate (MDI), polymeric methylene diphenyl diisocyanate (PMDI), p-phenylene diisocyanate (PDI), m-phenylene diisocyanate (PDI), naphthalene 1,5-diisocynate (NDI), naphthalene 2,4-diisocyanate (NDI), p-xylene diisocyanate (XDI), and homopolymers and copolymers thereof. The aromatic isocyanates are able to react with the hydroxyl or amine compounds and form a durable and tough polymer having a high melting point. The resulting polyurethane or polyurea generally has good mechanical strength and cut/shear resistance. However, one disadvantage with using aromatic isocyanates is the polymeric reaction product tends to have poor light stability and may discolor upon exposure to light, particularly ultraviolet (UV) light. Because aromatic isocyanates are used as a reactant, some aromatic structures may be found in the reaction product. UV light rays can cause quinoidation of the benzene rings resulting in yellow discoloration. Hence, UV light stabilizers are commonly added to the formulation, but the covers may still develop a yellowish appearance over prolonged exposure to sunlight. Thus, golf balls are normally painted with a white paint and then covered with a transparent coating to protect the ball's appearance.
In a second approach, aliphatic isocyanates are used to form the prepolymer. Examples of aliphatic isocyanates include, but are not limited to, isophorone diisocyanate (IPDI), 1,6-hexamethylene diisocyanate (HDI), 4,4′-dicyclohexylmethane diisocyanate (“H12 MDI”), and homopolymers and copolymers thereof. These aliphatic isocyanates can provide polymers having good light stability but such polymers tend to have reduced mechanical strength and cut/shear-resistance.
As discussed above, golf ball covers having good light stability are needed. One objective of this invention is to develop a golf ball cover having good light stability that does not sacrifice important mechanical properties such as high tensile strength and cut/tear-resistance. It is also desirable that the golf ball cover be made of a tough and durable material that can withstand high temperatures for significant periods of time. Another objective of this invention is to develop a golf ball having high thermal stability. When polyurea and polyurea/urethane hybrid compositions are used as the cover materials, the properties of the composition depend in significant part upon the components or building blocks used to make the compositions, particularly the isocyanates, polyamines, and curing agents. It would be beneficial to develop isocyanate blends that could provide the polyurea and polyurea/urethane compositions with such desirable properties as high tensile strength, impact durability, cut/tear-resistance, light stability, and thermal stability. One objective of this invention is to develop such isocyanate blends. The present invention provides golf ball cover materials having such characteristics as well as other advantageous properties, features, and benefits.